Nanostructured devices from ceramic nanomaterials

ABSTRACT

Nanostructured devices with a domain size less than 500 nanometers and low cost manufacturing methods for preparing these are provided. Applications of nanostructured binary oxides, ternary oxides, quaternary oxides, polyatomic forms of oxides, carbides, nitrides, borides, chalcogenides, halides, silicides and phosphides.

RELATED APPLICATIONS

[0001] This application is a divisional of co-pending U.S. patentapplication Ser. No. 10/001,660 titled “REDUCING MANUFACTURING AND RAWMATERIAL COSTS FOR DEVICE MANUFACTURE WITH NANOSTRUCTURED POWDERS” filedon Dec. 3, 2001 which is a divisional of U.S. Pat. No. 6,514,453 whichwas filed on Feb. 17, 1998 and claims the benefit of an earlier filedprovisional application serial No. 60/062,907 entitled “Thermal SensorsPrepared from Nanostructured Powders” which was filed on Oct. 21, 1997which are all incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] It is desirable for many different applications to monitor thetemperature and the changes in the temperature of a medium. Inparticular, rapid monitoring of such changes is necessary and evenrequired for many applications. For example, in engine environmentmonitoring and biomedical events monitoring, a response time of lessthan 5 seconds, or preferably less than 1 second is desirable.Applications requiring monitoring of radiation, power, heat and massflow, charge and momentum flow, and phase transformation also demandrapid response times. Faster response times are, in general, thepreferred performance even in applications that currently use devicesthat offer very slow response. For example, in ultra-precisionmanufacturing, temperature control is by far one of the most convenientmethods of objective control structure. In machining at high speeds, thetemperature of the tool or the substrate is a critical indicator ofmanufacturing efficacy; similarly welding, casting, milling,electrodischarge machining, chemical or laser etching of screens andstencils, bonding of dissimilar materials, lathe motor windingtemperature, and related manufacturing equipment and processes are allthermally intensive, and the rapid sensing and control of temperature iscritical to the end product quality. The response time of the thermalsensor determines the efficacy and the effectiveness of temperaturecontrol equipment for many applications including the monitoring ofcoolant and lubricant temperature before, during, and after an engine ormotor operation, medical applications, icing of wings, phasetransformations caused by physical or chemical effects, compositiontransformations caused by physical or chemical effects, the monitoringof pollution prevention units, exhausts, heaters, ovens, householdapparatus, laboratory and industrial instruments, furnaces, and finallyfire/heat detection and prevention systems.

[0003] The temperature of a medium is commonly monitored over a rangeusing devices based on thermocouples, RTDs or thermistors.Thermocouples, by far the most common technique, are unsatisfactory inmany applications as their response time is slow and often in the rangeof 30 to 500 seconds. RTDs are faster, however they are alsounsatisfactory for applications that require rapid monitoring becausetheir response time is 20 to 50 seconds even at higher temperatures. Ofthe known devices, thermistors are the best in their response times, butthey are still limited to response times in the range of 5 to 25seconds.

[0004] Thermistors are thermally sensitive resistors used in a varietyof applications, including temperature measurement A thermistor is apiece of semiconductor made from metal oxides, pressed into a smallbead, disk, wafer, or other shape, sintered at high temperatures, andfinally coated with epoxy or glass. The resulting device exhibits anelectrical resistance that varies with temperature. The two types ofthermistors include: negative temperature coefficient (NTC) thermistors,whose resistance decreases with increasing temperature, and positivetemperature coefficient (PTC) thermistors, whose resistance increaseswith increasing temperature. NTC thermistors are much more commonly usedthan PTC thermistors, especially for temperature measurementapplications.

[0005] A main advantage of thermistors for temperature measurement istheir high sensitivity. For example, a thermistor can have a sensitivitythat is 10 or more fold higher than platinum-based RID which itself isabout 3 to 10 fold more sensitive than thermocouples. The physicallysmall size of the thermistor bead can also help yield a very fastresponse to temperature changes.

[0006] Another advantage of the thermistor is its relatively highresistance. Thermistors are available with base resistances (at 25° C.)ranging from hundreds to millions of ohms. This high resistancediminishes the effect of inherent resistances in the lead wires, whichcan cause significant errors with low resistance devices such as RTDs.For example, while RTD measurements typically require 3-wire or 4-wireconnections to reduce errors caused by lead wire resistances, 2-wireconnections to thermistors are usually adequate. The major tradeoff forthe high resistance and sensitivity of the thermistor is its highlynonlinear output and relatively limited operating range.

[0007] One drawback of thermistors, however, is their use over limitedtemperature ranges. Thermistors have been used primarily forhigh-resolution measurements over limited temperature ranges, and oneexample of such an application is medical thermometry.

[0008] Another drawback to the use of thermistors is that, because oftheir small size and high resistance, they are prone to self-heatingerrors. When current is passed through the thermistor, power dissipatedby the thermistor, equal to I²R, will heat the thermistor. Manufacturerstypically specify this as the dissipation constant, which is the powerrequired to heat the thermistor 1° C. from ambient temperature (mW/C.).The dissipation constant depends heavily on how easily heat istransferred away from the thermistor, so the dissipation constant may bespecified for different media. This phenomenon is the basis ofapplication of thermistor devices for monitoring of power, heat and massflow, of charge and momentum flow, and of phase transformation.Nevertheless, a stable and reproducible dissipation constant is requiredin various applications; a requirement which state of the artthermistors usually fail to offer.

[0009] In summary, the slow response time, limited temperature range,the high thermal mass, the self-heating errors are the most importantlimitations of thermistors. This invention teaches a method ofovercoming these limitations. Although this invention describes NTCthermistors, it would be obvious to those skilled in the art that therationale and method discussed applies to practice of PTC thermistors aswell. Furthermore, the rationale and method described in detail lateralso offers practical insights for the design and practice of superiorRTDs and thermocouples as well. The teachings can be used to developsuch devices that are superior in response characteristics, sensitivity,resistivity, stability, miniaturization, thermal mass, sinteringtemperature, electrode costs, and sintering time. Finally, while it isconventional to use a thermistor's resistance measurement fortemperature monitoring, this invention's teachings can also be easilyextended to any electrical property of a thermal sensor, including butnot limited to capacitance, inductance, impedance, conductance,admittance, and loss factor.

SUMMARY OF THE INVENTION

[0010] Briefly stated, the present invention involves quantum confineddevices and methods of preparing such devices from nanostructuredpowders.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 depicts the effect of precursor size on the resistor layerthickness.

[0012]FIG. 2 depicts an illustrative method of preparing a thermalsensor device.

[0013]FIG. 3 depicts a comparison of the preparation of a conventionaldevice prepared from micron sized precursors with the device of thepresently claimed invention which uses nano-precision engineeredprecursors.

[0014]FIG. 4 depicts device architectures for thermistors.

[0015]FIG. 5 depicts the parallel printing of sensor electrodes.

[0016]FIG. 6 depicts a sampler's electrode enlarged.

[0017]FIG. 7 depicts the method of rapidly monitoring the thermal state.

[0018]FIG. 8 depicts a thermistor in an illustrative circuit to measuretemperature.

[0019]FIG. 9 depicts a typical thermistor bolometer detector circuit.

[0020]FIG. 10 depicts a flowchart for the preparation of nanoscalepowders via chemical precipitation.

[0021]FIG. 11 depicts an XRD pattern of nanoscale (Ni, Mn)₃O₄ displayingpeak broadening effects.

[0022]FIG. 12 depicts an XRD pattern of micron (Ni,Mn)₃O₄ calcined at850° C. for 4 hours.

[0023]FIG. 13 depicts the resistance-temperature characteristics forthermistors formed from micron-scale powders, fired at 1075° C.

[0024]FIG. 14 depicts the resistance-temperature characteristics forthermistors formed from nano-scale powders, fired at 1075° C.

[0025]FIG. 15 depicts the characteristics response of a micron-scalethermistor, BETA of 3730 K.

[0026]FIG. 16 depicts the characteristics response of a nanoscalethermistor, BETA of 6140 K.

[0027]FIG. 17 depicts the performance of nanoscale (YSr) (CrFe)O₃ basedthermal sensor.

[0028]FIG. 18 depicts the screen-printed thermal sensor array on anAg—Pd electrode.

[0029]FIG. 19 depicts the resistance temperature characteristics for athermistor array displaying a high BETA value of 4923 K.

[0030]FIG. 20 depicts an interdigital device composed of SnO₂ displayinga negative change in resistance with temperature.

[0031]FIG. 21 depicts the resistance-temperature characteristics for athick film thermistor element on interdigital electrodes with a BETAvalue of 4910 K.

[0032]FIG. 22 depicts the response of a high temperature sensor.

DETAILED DESCRIPTION OF THE INVENTION

[0033] [This paragraph only contains relevant material from the summaryof invention section of the '453 parent application] In general, thepresent invention involves method of preparing a device comprisingforming a material using nanostructured powders, forming a structurefrom the material that can support its own weight and retain its shapeeven when the environment changes, and electroding the device. In oneembodiment, the method further comprises sintering the sensing materialto increase the material's density and structural strength. This stepcan be performed either before formation of a structure from thematerial or can be performed after the step of electroding the device.

[0034] The presently claimed invention describes a method of rapidlymonitoring the temperature of a medium and a method of preparing aquantum confined device that can enable such measurements. Specifically,the electrical properties or changes in electrical properties, such asimpedance, of nanostructured thermal sensor devices is measured. Thepresently claimed invention also describes a device for rapidlymonitoring changes in the temperature of a medium and a method forpreparing a device for rapidly monitoring changes in the temperature ofa medium. The invention can be used to monitor the absolute value of andchanges in temperature of gases, inorganic and organic liquids, solids,suspensions and mixtures of one or more of the said phases. Theinvention can be used to monitor radiation, power, heat and mass flow,charge and momentum flow and phase transformations. The materialcompositions to be used in the presently claimed invention arenanostructured materials, i.e. materials whose domain size have beenengineered to sub-micron levels, preferably to a dimension where sizeconfinement effects become observable, and thus the electrical orthermal or both properties of the materials are modified.

[0035] Nanostructured materials (nanomaterials) are a novel class ofmaterials whose distinguishing feature is that their average grain sizeor other structural domain size is below a critical characteristiclength. In case the characteristic length is unknown or difficult todetermine, a good rule of thumb is to use 500 nanometers as thecharacteristic length, and more preferably 100 nanometers as thecritical characteristic length. Within this size range, a variety ofconfinement effects dramatically change the properties of the material.A property will be altered when the entity or mechanism responsible forthat property is confined within a space smaller than the criticallength associated with that entity or mechanism. The importance ofnanostructured materials to this invention can be appreciated byconsidering the example of the mean free path of electrons, which is akey determinant of a material's resistivity. The mean free path inconventional materials and resistivity are related by:

ρ=mv _(E) /nq ²λ

[0036] where,

[0037] ρ: resistivity

[0038] m: mass of electron

[0039] v_(E): Fermi energy

[0040] n: number of free electrons per unit volume in material

[0041] q: charge of electron

[0042] λ: mean free path of electron

[0043] This equation assumes that the resistivity in the material isdetermined in part by the mean free path of electrons and that theelectrons have a free path in the bulk. In nanostructured materials, thedomain size is confined to dimensions less than the mean free path andthe electron meets the interface of the domain before it transverses apath equal to the mean free path. Thus, if the material's domain size isconfined to a size less than the mean free path, this equation is nolonger valid (in a simplistic model, one could replace A with the domainsize, but that replacement ignores the fact that confinement can alsoaffect “n” and other fundamental properties). This insight suggests thatunusual properties may be expected from devices prepared from materialswith a domain size less than the mean free path of electrons. While theabove argument is discussed in light of mean free path, it is importantto note that the domain confinement effect can be observed even when thedomain size is somewhat larger than the mean free path because: (a)“mean” free path is a statistical number reflecting a mean of pathlengths statistically observed in a given material and, (b) in very finematerials, the interface volume is significant and all the freeelectrons do not see the same space, electrons closer to interfaceinteract differently than those from the center of the domain. In thepresently claimed invention, the devices are prepared fromnanostructured materials with a domain size less than 5 times the meanfree path of electrons in the given material, preferably less than themean free path of electrons. In the event that it is difficult totheoretically compute the mean free path of the material underconsideration, it is recommended that the domain size be less than 500nanometers, and most preferably less than 100 nanometers.

[0044] In one aspect of the invention, a thermal sensor device isprepared from the abovementioned nanomaterials. For thermal sensors, thesignificance of using nanostructured materials can be furtherappreciated if the conductivity of semiconducting oxides is consideredas shown in the equation for conductivity from hopping mechanism:$\sigma = {P_{a} \cdot P_{b} \cdot \frac{2e^{2}}{ckt} \cdot v \cdot {\exp ( {- \frac{q}{kT}} )}}$

[0045] P_(a), P_(b): probabilities that neighboring sites are occupiedby desirable cations

[0046] e: electronic charge

[0047] n: frequency factor

[0048] k: Boltzmann's constant

[0049] T: temperature

[0050] q: activation energy

[0051] c: unit cell dimension

[0052] The frequency factor and activation energy are a strong functionof the microstructure confinement and therefore the conductivity of thesame material can be very different in nanostructured form when comparedwith micron sized form.

[0053] Device miniaturization is also a significant breakthrough thatthe presently claimed invention offers through the use of nanostructuredmaterials. Existing precursors that are used to prepare thermistors arebased on micron-sized powders. The thermal mass of the sensor is in partdependent on the powder size. FIG. 1 outlines the problem. As can beseen in FIG. 1, the layer thickness cannot be less than a few multiplesof the precursor powder size. With nanostructured powders, the thermalsensor's active element size and therefore its thermal mass can bereduced significantly. For example, everything else remaining the same,the thermal mass of a thermistor can be reduced by a factor of 1000 if10 nanometer powders are used instead of 10 micron powders. This methodof reducing thermal mass is relevant to thermocouples, RTDs,thermistors, and devices where thermal mass is an important performancedeterminant. The presently claimed invention teaches that nanostructuredpowders are preferred to reduce the thermal mass of a device.

[0054] Preparation of Thermal Sensor Device

[0055] An illustrative method of preparing said device is shown in FIG.2. The impedance of the device produced is capable of changing with thethermal state of or around the device. As a corollary, this inventionrequires that the device have a finite and detectable impedance in itsinitial as produced state. The impedance can be because of finite anddetectable electrical property such as but not limiting to finite anddetectable resistance, finite and detectable capacitance, finite anddetectable inductance, or a combination of such properties.

[0056] The steps involved in forming the thermal sensor device includethe formation of a thermal sensing material using nanostructuredpowders, formation of a structure that can support its own weight andretain its shape even when the environment changes, and finallyelectroding the structure. In an optional step, the sensing material canbe sintered to increase the material's density and structural strengthprior to or after the step of electroding the sensing material.

[0057] As discussed above, the material compositions to be used in thepresently claimed invention are nanostructured materials, i.e.,materials whose domain size have been engineered to sub-micron levels,preferably to nanoscale levels (i.e., less than 100 nanometers) wherequantum confinement effects become observable. FIG. 3 illustrates theadvantages of utilizing nano-precision engineered precursors in themethod of the presently claimed invention.

[0058] The device can be produced from various thermally sensitivematerial compositions which include ceramics, metals and alloys,polymers, and composites. The ceramics include but are not limited tobinary, ternary, quaternary, or polyatomic forms of oxides, carbides,nitrides, borides, chalcogenides, halides, silicides, and phosphides.Most preferably, the ceramics are oxides. The invention also includesstoichiometric and non-stoichiometric forms of ceramics, undoped anddoped forms of ceramics, and different phases of the same composition.The ceramics used, however, are limited to ceramics that have a finiteand detectable impedance and does not include ceramics and their formsthat have an impedance higher than those currently detectable.

[0059] Metals and alloys such as those formed from a combination of twoor more of s group, p group, d group and f group elements. The inventionincludes stoichiometric and non-stoichiometric forms of alloys, undopedand doped forms of metals and alloys, and different phases of the samecomposition. The metals and alloys, however, are limited to metals andalloys that have a finite and detectable impedance, and does not includemetals and alloys or their forms that have impedance higher than thosecurrently detectable.

[0060] Polymers including but not limited to those with functionalgroups that enhance conductivity. Specific examples include but are notlimited to conducting polymers and ion-beam treated polymers. One ofordinary skill in the art will realize that other polymers such as metalfilled polymers or conducting ceramic filled polymers can also be used.The polymers used, however, are limited to polymers or their forms thathave a finite and detectable impedance and does not include polymers ortheir forms that have an impedance higher than those currentlydetectable.

[0061] Composites including but not limited to those formed from two ormore metals, alloys, ceramics, or the polymers discussed above may alsobe used. Examples of illustrative composites include but are not limitedto oxide-carbide composites, oxide-polymer composites, metal filledpolymer composites, nitride-alloy composites, oxide-carbide-polymercomposites. One of ordinary skill in the art will appreciate that othercomposites can also be used such as defect engineered composites. Thecomposites are limited to composites that have a finite and detectableimpedance and does not include composites that have an impedance higherthan those currently detectable. While any of, these materialcompositions can be used, it is preferred that those compositions beselected for device applications that provide log linear but high slopedvoltage-current characteristics and resistance-temperaturecharacteristics. In particular, compositions are preferred that yield ahigh value of the material constant (BETA value, beta value) asdetermined from:

β=ln(R ₁ /R ₂)/(l/T ₁-l/T ₂)

[0062] In this invention, a β value greater than 10 is desirable, above100 is preferred, above 1000 is more preferred, and above 10,000 is mostpreferred.

[0063] An additional selection rule for the desired composition is theTemperature Coefficient of Resistance (TCR), α, which is defined for anymaterial as the ratio of the rate of the change of resistance withtemperature to the resistance at a specified temperature as depictedmathematically below:

α=(1/R)·(dR/dT)

[0064] where, R is the resistance of the thermistor at temperature T.For thermistors α is normally expressed in units of % per ° C. In thepresently claimed invention, an α value greater than 0.01% per ° C. isdesirable, above 0.1% per ° C. is preferred, above 1% per ° C. is morepreferred, and above 10% per ° C. is most preferred.

[0065] Additionally, it is also preferred that the device containceramic compositions and it is more preferred to prepare devicescontaining oxide ceramic compositions as one of the constituents. It ismost preferred to prepare devices containing oxide ceramic compositionsbased on one or more of the following elements: Ti, Mn, Fe, Ni, Zn, Cu,Sr, Y, Zr, Ta, W Sc, V, Co, In, Li, Hf, Nb, Mo, Sn, Sb, Ce, Pr, Be, Np,Pa, Gd, Dy, Os, Pt, Pd, Ag, Eu, Er, Yb, Ba, Ga, Cs, Na, K, Mg, Pm, Pr,Bi, Tl, Ir, Rb, Ca, La, Ac, Re, Hg, Cd, As, Th, Nd, Tb, Md, and Au, Al,Si, Ge, B, Te, and Se.

[0066] Once the appropriate nanomaterial composition has been selected,the thermal sensing material can be shaped in various forms includingbut not limited to a film, coil, rod, fiber, sphere, cylinder, bead,pellet, non-uniform shape or combination thereof. These shapes may alsobe in solid or hollow form, in monolithic or integrated form, insingular to array form, on non-flexible or no substrate, on inorganic ororganic substrate. The thermal sensing material can be shaped into thesevarious forms using one of the following manufacturing methods or acombination thereof including pressing, extrusion, molding, screenprinting, tape casting, spraying, doctor blading, sputtering, vapordeposition, epitaxy, electrochemical or electrophoretic deposition,thermophoretic deposition, centrifugal forming, magnetic deposition, andstamping. The sensing material can be porous or dense, thin or thick,flat or curved, covered with a barrier or exposed.

[0067] In an additional aspect of the presently claimed method asubstrate may be used, however, it is not required. It is necessary,however, to form a structure, as described above. In the event asubstrate is used or preferred, the substrate on which electrodes areformed can be flat or curved, flexible or rigid, inorganic or organic,thin or thick, porous or dense. The preferred substrates are those thatprovide the mechanical properties needed for device life greater thanthe anticipated device usage life. The substrate should have sufficientstrength and toughness to support its own weight and the weight of thedevice prepared on the device. The substrate should have thermal andmechanical shock resistance, i.e. sufficient strength and toughness towithstand the thermal stress (e.g. from expansion) or mechanical stress(e.g. vibration). The substrate should also have hardness andenvironmental resistance to prevent degradation of the device or itselfor both over the expected usage of life of the device. For selectivity,one may integrate a resistive element in the substrate to warm thesensor to a desirable temperature to facilitate the interaction of theenvironment of interest during start up of the device. However, if theheating element is integrated, care must be taken to avoid shorting ofthe electrode or transmission of noise from the resistive element to theimpedance signal from the electrode.

[0068] In the final step of the presently claimed method, it is requiredthat the device be electroded. The electrode can be a wire or plate orcoil, straight or curved, smooth or rough or wavy, thin or thick, solidor hollow, and flexible or non-flexible. For device designs that preferno substrate including but not limited to bead/pellet type devicedesigns, it is preferred that the thermal sensor is formed directly onthe electrode wire or plate or coil. It is important in all cases thatthe electrode be conductive and stable at the usage temperatures. It ispreferred that the electrode composition does not react with the sensingmaterial or the environment during the manufacture or use of the device.It is also preferred that the melting point of the electrode is higherthan the highest temperature to be used during the manufacture or use ofthe device. FIG. 4 illustrates some examples of device architecture forthermistors. One of ordinary skill in the art will realize that otherdevice architectures can also be used in the presently claimedinvention.

[0069] In one example, a sensor electrode can be built by a parallelscreen printing technique. This approach has the advantage that itprints many sensors in parallel which can help dramatically reduce thecost per sampler element. As shown in FIG. 5, a mask layer is firstformed. FIG. 6 shows one of the electrode structures enlarged. Theinterdigitated pattern is chosen because the structure provides themaximal area for quick response. The structure is also inherently robustgiven the fact that even if one or more electrode fingers fail thesensor will still be functional. The edges of each electrode finger arerounded to minimize edge voltage related noise. The electrodes for thesampler are first prepared by obtaining a screen-printing paste. Thepaste is then placed on a screen mask and imprinted onto the substrateusing a squeegee. A semi-automatic screen printer is available toperform this task. The electrode layer is then dried and fired at 500°C. to remove any organics in the electrode. The nanostructured activesensing layers can then be deposited as described in Example 10. Firstnanoceramics are homogenized with screen printing pre-mix (cellulose,cyclohexanone). The thermistor films are next printed using the screenprinting approach described above. Once printed, the films are thendried and then cured at 200° C. to eliminate any traces of organics inthe film. Finally the nanopowders layer is sintered at 850° C. for 2hours to stabilize the NTC ceramic film. The time and the temperaturesfor this thermal treatment can be varied to identify the conditions thatminimize grain growth, provide robust and dense films, and best producethermistor arrays with reproducible performance.

[0070] Additionally, in an optional step in the presently claimedmethod, the sensing material can be sintered to increase the material'sdensity and structural strength. The sintering step can be performedbefore or after electroding of the substrate occurs. The sintering canbe accomplished in an open or closed heating apparatus, in an oxidizingor reducing or inert environment, in flow or non-flow type heatingapparatus, in a system that applies heat conductively or convectively orradiatively or a combination of these techniques, and with or withoutthe application of external pressure during or before the application ofheat. Some or all of the heat needed for sintering may be provided byinternal heat such as those released during solid-state combustion ofmaterials within the device.

[0071] The presently claimed invention also advantageously allows thedevice sintering temperature to be lowered. For example, if the deviceis utilizing platinum as the electrode material for the thermal sensorbecause of the high sintering temperature of the sensor's activematerial, then one approach of enabling the use of lower melting pointmetals or alloys for electrodes is by reducing the sintering temperatureof the active material.

[0072] The densification of a powder compact or film, or the sinteringof a powder compact or film, is essentially a process of removing thepores between the starting particles, combined with growth and strongbonding between adjacent particles. The driving force for densificationis the free-energy change, or more specifically, the decrease in surfacearea and lowering of the surface free energy by the elimination ofsolid-vapor interface. Among the processing variables that may affectthe densification process, the particle size of the starting powder isone of the most important variables. In solid-state processes, assumingthat the matter transport is controlled by lattice diffusion, thedensification rate or the volume change of the powder compact duringsintering can be related to processing variables as follows:$\frac{\Delta \quad V}{V_{0}} = \lbrack {3{( \frac{20 \cdot \gamma \cdot a^{3} \cdot D^{0}}{\sqrt{2} \cdot {kT}} ) \cdot r^{- 1.2} \cdot t^{0.4}}} \rbrack$

[0073] where:

[0074] V₀: the initial volume of the powder compact,

[0075] AV: the change of the volume during densification;

[0076] T: the sintering temperature

[0077] t: the sintering time

[0078] k: the Boltzman constant,

[0079] D⁰: the self-diffusivity,

[0080] y: the surface energy of the particle,

[0081] a³: the atomic volume of the diffusing vacancy,

[0082] r: the radius of the particle of the starting powder

[0083] As can be seen from the equation, the densification rate isroughly proportional to the inverse of particle size. Given the samesintering temperature and starting material, the densification rate canbe increased 1000 fold by using 10 nanometer-sized powders instead of 10micron sized powders. Alternatively, to obtain the same densification,the sintering can be conducted at lower temperatures starting withsmaller sized powders. Thus, the use of nanoscale powders for sensingmaterials can also significantly decrease the sintering temperaturescurrently necessary for micron-sized powders. The decrease is greaterthan at least 100° C., more often between 100 and 500° C., and sometimesgreater than 500° C., Additional advantages include the energy savingsfrom lower processing temperatures and the reduction of processing timessuch as inventory costs.

[0084] An additional advantage is that because of the high surface areaand high diffusivity of nanoscale powders, they may be sintered withoutimpurity inducing sintering aids. The resulting product is thus morereliable and reproducible and have enhanced service temperatures andhigh temperature strength.

[0085] Use of the Thermal Sensor Device

[0086] The presently claimed invention also describes a method formonitoring thermal state changes using a nanostructured thermal sensordevice involving the steps of measuring the electrical property or thechange in the electrical property using an electrical sensor deviceprepared from nanostructured materials and correlating the measurementof the electrical property to the thermal state. FIG. 7 depicts theapproach of the presently claimed invention. The invention can be usedto monitor absolute changes in the temperature of gases, inorganic andorganic liquids, solids, suspensions and mixtures of one or more of thesaid phases. FIG. 8 shows an illustrative circuit that uses a thermistoras a temperature sensing device. Referring to FIG. 8, a potential “E” isapplied to the wheatstone bridge which at a set point is balanced. Thethermistor resistance and displayed galvanostat reading are calibratedat various temperatures. In an actual operating environment, any changein the thermistor temperature changes the thermistor's resistance whichdisturbs the bridge balance which is sensed and displayed. A simplecalibrated ammeter or voltmeter can also be used to measure theresistance and calibrate the resistance to temperature of the thermistorcorresponding to the measure thermistor resistance. The preferred methodis the bridge method.

[0087] The presently claimed invention can also be used to monitorradiation, power, heat and mass flow, charge and momentum flow, andphase transformation. FIG. 9 presents an example where the thermistor isused in a bolometer detector circuit. The active and compensatingthermistors are part of a bridge circuit as shown. Equal bias voltagesof opposite polarity with respect to ground are applied across theelements to minimize noise and microphonics. With no radiation on thedetector, the bridge remains unbalanced. Absorption of radiation causesa temperature rise, hence a resistance decrease in the detector elementcausing the bridge to become unbalanced. The out-of-balance signal isamplified and translated into an indicator and recorded. Applications ofthermistor bolometers include radiometers, pyrometers, andnon-destructive examination (NDE) of products.

[0088] For simplified signal processing, it is preferred that impedancemeasurements be performed at a single frequency or with d.c. Forreliability, an array of sensing elements can be used, all of themworking at the same frequency, or some of them working at differentfrequencies, or all of them working at different frequencies.Furthermore, the material of composition or operating temperature ofeach array element may be different. An array of elements can enable themeasurement of temperature profiles over a space for a specified time oras a function of time.

[0089] In particular, the presently claimed invention can be used tomonitor temperature changes while machining at high speeds, welding,casting, milling, electrodischarge machining, chemical or laser etchingor screens and stencils, bonding of dissimilar materials, lathe motorwinding temperature and other thermally intensive manufacturingequipment and processes. Similarly, the coolant and lubricanttemperature before, during and after an engine or motor operation can bemonitored, as well as monitoring temperature during the icing of wings,in medical applications, in phase transformation caused by physical orchemical effects, in composition transformations caused by physical orchemical effects, in the monitoring of pollution prevention units,exhausts, heaters, ovens, household apparatus, laboratory and industrialinstruments, in furnaces, and in fire/heat detection and preventionsystems.

[0090] Although the abovementioned examples are representative of thechanges in temperature that can be monitored, one of ordinary skill inthe art will appreciate that any changes in temperature can bemonitored, as well as the absence of changes in temperature. It willalso be appreciated that the presently claimed device can be used torapidly monitor changes in radiation, flow of medium, and the state ofthe medium, as well as the absence of these changes.

[0091] It is preferred that the presently claimed invention rapidlymonitor the above mentioned changes; in particular, is preferred thatthese changes are monitored in less than 5 seconds. It is most preferredif these changes are monitored in less than 1 second.

[0092] Applications of the Thermal Sensor Device

[0093] Sensors form the foundation for all control technologies anddefine the architecture, the layout, the performance, and thereliability for the control technology. The presently claimed thermalsensors enable more efficient, lower pollution, higher performanceenvelop operation of propulsion and power systems for combat vehicles,aircraft, and logistic support infrastructure. The fuel savings fromdynamically controlled engines translates into significantly reducedcosts per mission, lower weight for the same mission, more compactequipment, higher thrust to weight ratio, and greater range.

[0094] The presently claimed invention also has excellent potential forcommercialization. For example, ultra-precision manufacturing requiresthe ability to rapidly detect, monitor, and evolve processingconditions. Temperature sensing is an excellent alternative and iscurrently used, albeit with slow response. Nanostructured thermistorswill enable rapid response temperature sensing. Processes that willbenefit include processes such as machining at high speeds, because thetemperature of the tool or the substrate is a critical indicator ofmanufacturing efficiency; similarly improvements in welding, casting,milling, electrodischarge machining, chemical or laser etching ofscreens and stencils, and bonding of dissimilar materials will beimproved.

[0095] Additionally, the miniature size of the nanostructuredthermistors is ideally suited to detect small temperature differences.Thus, accurate temperature measurements can be made using the presentlyclaimed thermistors.

[0096] Thermistor bolometers are another application of the presentlyclaimed invention. Thermistor bolometers are used for infra-reddetection in radiometers, pyrometers, automatic product analysis,spectroscopy, and automatic process control. Potential applications alsoexist in biomedical devices for thermal imaging, non-invasive patientcare, and post-surgical monitor of skin grafts and organ transplants.Further applications include thermography for insulation check andstructural integrity check of old buildings, bridges and homes. In spaceprograms and missiles, the proposed technology offers the neededadvantage of miniaturization and power-needs over cooled photodetectors.

[0097] Furthermore, another application is for microwave powermeasurement. As the frequency of an electromagnetic field increases,current and voltage measurements are impractical and error prone giventhe fact that the dimensions of the measurement instruments become asignificant fraction of the wavelength of the electromagnetic field.Instead, power and impedance measurements are more appropriate. Theproposed technology is perfectly suited for this application. As amicrowave thermistor, the nano-engineered device proposed will enablefast response, broadband, high detectivity, and miniatureinstrumentation.

[0098] The currently described technology also has direct applicationsfor power calorimetry in measuring particle beams because of theanticipated enhancements in sensitivity.

[0099] Additionally, the presently claimed invention can be used forliquid and gas flow detectors. The mass flow changes the environment ofthe thermistor and therefore the dissipation constant. The enhancedsensitivity of the proposed thermistors will enable the detection ofsmall changes in liquid or gas flow. This can enable reliable flowdetectors for process safety and control.

[0100] Finally, the presently claimed invention can also be used as aphase sensor. Once again, as described above, the change in theenvironment changes the thermal boundary conditions and the proposedthermistors can therefore be used to detect changes in phase (solid toliquid to gas), level of liquids, ice formation on walkways, bridges,and freezing of water lines to name a few.

[0101] In summary, the presently claimed invention enables themeasurement of temperature, radiation, flow of a medium, and state of amedium. Additionally, changes or the absence of changes in temperature,radiation, flow of a medium, and the state of a medium can be monitored.

[0102] The presently claimed invention is described in detail in thefollowing examples, however these examples are not intended to limit thescope of the invention.

EXAMPLES 1 THROUGH 8 Preparation of Sensors from (Mn,Ni)₃O₄ with TiO₂

[0103] Nanoscale nickel-manganate powders were synthesized via a wetchemical precipitation method as outlined in FIG. 10. The startingcomposition mix of the precursors was adjusted to achieve a formulationof Ni₀₆₄Mn_(2.36)0₄, Nickel chloride is first put into solution underslight heat and stirred with a magnetic stir bar. Concentrated nitricacid was added until all hydroxide formed during the nickel chlorideaddition was completely dissolved. Manganese chloride was also put intosolution and the two were mixed together and then slowly poured into oneliter of a 1.25 molar solution of sodium hydroxide and a precipitateformed. The precipitate is then rinsed with distilled water in order toremove the aqueous salt solution while vacuum filtering in a 4000 mlfilter flask. It was next dried and pulverized, and finally calcined inan alumina crucible. The color of the precipitate before it is dried isa greenish rust color and on drying becomes black.

[0104]FIG. 11 is an XRD pattern from the precipitate powder illustratingthe peak broadening associated with small crystalline size. The meancrystalline size (from Scherrer analysis) was 26 nm. BET analysis of thechemically produced nickel-manganate powders gave a specific surfacearea of 112 m²/g. The composition at this point was considered to be anamorphous hydroxide, and calcination was done at 500° C. to crystallizethe powder.

[0105] A standard oxide route was also used to make micron-sized powderof the same composition. Nickel(II) oxide (Aldrich) powders were used.The composition NiO_(0.64)Mn_(2.36)0₄ was batched accordingly and thenball-milled with zirconia media for 24 hours. The powder was dried at80° C. and then calcined at 850° C. for four hours. FIG. 12 shows theXRD pattern for the calcined powder containing a spinel phase(Ni,Mn)₃O₄. A low value of specific surface area of 1.5 m²/g was asexpected. The calcined powders were then charged into a metal die toproduce pellets having a diameter of 6.4 mm and thickness of 2 mm. Thepellets were then sintered in atmospheric air at 600, 800, 1000 and1200° C. for one hour to produce thermistor elements.

[0106] Nanoscale TiO₂ was added to the (Mn₅Ni)₃O₄ composition tofunction as a sintering aid and to prevent cracking of the firedcomponents. As TiO₂ is an insulating ceramic, it has the inherentdrawback of raising the electrical resistivity and a satisfactory amountwas determined empirically to be 5%. The same molar amount ofmicron-scale TiO₂, 5%, was also added to the micron scale composition tohave comparative samples with which to relate resistance temperaturecharacteristics of nanostructured materials based thermistor and micronpowders based thermistor.

[0107]FIGS. 13 and 14 are the respective resistance temperature pilotsfor micron-and nanoscale thermistors fired to 1075° C. The nanoscalethermistor exhibits an extremely large change in resistance over arelatively short temperature range which is what is needed for highsensitivity.

[0108] To obtain a quantitative comparison, these plots are thenlinearized with a natural log resistance versus the inverse oftemperature graph and are shown in FIGS. 15 and 16. The steepness of theslope indicates the rate at which the resistance changes. The value ofthe slope is also termed the Material Constant, or β value.

[0109] The raw data for compositions with and without TiO₂ additions,and fired at several different temperatures were analyzed in the abovemanner and the results are tabulated in Table 1 below: TABLE 1 MaterialFiring Constant Correlation Sample Temperature β (K) CoefficientResistivity (ohm-m) M7 1040 1040 3520 0.99995 278 M71075 1075 36300.99998 272 N7 1040 1040 3810 0.99761 980 N7 1075 1075 6140 0.988862,850 M8 1040 1040 3480 0.99806 54 M8 1075 1075 3590 0.99942 160 N8 10401040 7204 0.99682 8,210 N8 1075 1075 4160 0.99816 12,900

[0110] A remarkably high value of the material constant β exists foreach of the nanocompositions, with those being almost twice that of thesame composition thermistor formed from micron powders.

EXAMPLE 9 (YSr)(CrFe)O3 based Thermal Sensor

[0111] A nanoscale thermistor formulation based on the perovskite system(Y_(1−x)Sr_(x)) (Cr_(1−y)Fe_(y))O₃) was produced via a wet chemicalprecipitation reaction from the constituent metal nitrates when x=0.05and y=0.20. In a 600 ml beaker, 3.63 g yttrium nitrate hexahydrate,0.106 g strontium nitrate, 3.20 g chromium (III) nitrate, and 0.808 giron were dissolved in 150 ml of distilled water. A one molar solutionof sodium hydroxide was then added until the reaction was completed. Theprecipitate was then vacuum filtered and rinsed with distilled water andthen denatured alcohol. The precipitate cake was then dried at 100° C.for 12 hours and calcined at 500° C. for one hour. The powder was nextpressed to form pellets 6.40 mm in diameter by 5 mm in thickness. Theywere fired at 20° C./min to 600° C. for 0.5 hr, 55° C./min to 1100° C.for 0.5 hr., and 60° C./min to room temperature. The pellets were thentested electrically to determine their thermistor characteristics. FIG.17 presents the results. The results as shown in FIG. 17 indicate a highvalue which suggests that the thermal sensor prepared from nanoscaleceramic composites can enable the rapid sensing of high temperatures.

EXAMPLE 10 Thick Film Thermistor Fabrication

[0112] The thermistor arrays were fabricated by screen printing using asemi-automatic screen printer. The screens used were made from stainlesssteel having a mesh count of 200, a wire diameter of 0.0007 inches, abias of 45 degrees, and a polymeric emulsion of 0.0002 inches.Electrodes of Ag/Pd (Electro Science Laboratories #D-9633-G) were firstscreen printed on 96% alumina substrates (Accu-Tech Laser Processing,Inc.) having a thickness of 0.025 inches, and then fired in air at 850°C. for 10 minutes. A thermistor ink layer was next applied on top of theelectrodes. It was made from thoroughly mixing nano-scale thermistorpowders formulations and Electro-Science Laboratory, Inc, Screenprinting vehicle #400, The thermistor formulations were e of (Mn,Ni)₃O₄with and without additions of Ti and Cu. After printing, the ink isallowed to level for 10 minutes, is then dried at 350° C. for 12 minutesand then is fired at 850° C. for 12 minutes. The thick film thermistorsthat have been printed onto several electrode configurations.

[0113] Thermistor Array: The following electrode array pattern shown inFIG. 18 was prepared using the above screen printing approach.Resistance-temperature measurements of the arrays were taken toestablish a baseline. Values from four arrays, (16 thermistors total),were averaged, and the material constant calculated from the plot isshown in FIG. 19. The BETA values of the thermistors measured at roomtemperature had an average of 4923 K and a standard deviation of 30 K.Once again, the high beta value confirms that the thermal sensor is moresensitive.

[0114] Another way of stating these results is from the standpoint ofthe TCR value, or the alpha value, where

α=−β/T ²(%/C.)

[0115] At room temperature, therefore, a thermistor having a β of 4923 Kwould have an alpha value of −5.5%/C. which is extremely sensitive.

EXAMPLE 11 Preparation of Nanoscale SnO₂ Based Thermal Sensor

[0116] Nanoscale SnO₂ was screen printed over an interdigital pattern asexplained above with the substitution of nanoscale tin oxide instead ofthe powder mix used before. The resistance-temperature characteristicsare shown in FIG. 20 and the material constant has been determined fromthe In R versus the inverse of temperature plot in FIG. 21 to be 4910 K.This would correspond to a TCR of 1.65%/° C. at 250° C.

EXAMPLE 12 Preparation of Thermal Sensors from NanoscaleY_(0.9)Zr_(0.1)Oxide

[0117] Nanoscale Y_(0.9)Zr_(0.1) oxide powder was produced via thefollowing wet chemical precipitation reaction:

0.9Y(NO₃)_(3.6)H₂0+0.1ZrOCl_(2.8)H₂O+NH₄OH→0.5(Y_(0.9)Zr_(0.1))₂O₃

[0118] After precipitation, the material was rinsed three times with hotdistilled water. The powders were then dried at 100° C. for 12 hours,and then calcined at 500° C. for one hours. XRD analysis determined theaverage crystalline size to be about 9.4 nm. BET analysis establishedthe specific surface area to be 80.4 m²/g. The powders were thencompressed into pellets 6.4 mm in diameter by 2 mm in thickness. Theywere electroded with Ag—Pd, and wired with Ag wire for electricaltesting. FIG. 22 presents the response. A large beta value of 9292 wasobserved and the response was logarithmically linear from 200° C. toabout 1000° C. This suggests that nanoscale powders can help produce avery high sensitivity thermal sensor for high temperature applications.

We claim:
 1. A method of preparing a nanostructured device comprising:providing ceramic nanostructured powders; forming a nanostructureddevice comprising of the ceramic nanostructured powders, wherein thenanostructured device has a domain size of less than 500 nanometers; andwherein the forming step comprises processing said ceramicnanostructured powders under conditions that retain domain confinementeffects in the nanostructured device.
 2. The method of claim 1, whereinthe nanostructured device has a domain size of less than 100 nanometers.3. The method of claim 2, wherein said ceramic is selected from thegroup consisting of: binary oxides, ternary oxides, quaternary oxides,polyatomic forms of oxides, carbides, nitrides, borides, chalcogenides,halides, silicides and phosphides.
 4. The method of claim 2, whereinsaid ceramic comprises oxygen.
 5. The method of claim 2, wherein saidceramic is stoichiometric.
 6. The method of claim 2, wherein saidceramic is non-stoichiometric.
 7. A product comprising of thenanostructured device prepared by the method of claim 1.